The present invention relates to electrochemical energy storage devices. More particularly, the present invention relates to electrochemical energy storage devices including at least one polymer electrode, which has a bicontinuous interpenetrating network of nanometer scale.
Electrochemical storage devices, such as batteries, can be classified into either primary or secondary batteries. Primary batteries are not designed to be recharged and are not reusable after they have been fully discharged. Secondary batteries, however, are rechargeable after they have been fully discharged. One example of a commercially widely available primary battery is an alkaline battery, which typically includes a zinc anode of large surface area, a manganese dioxide cathode of high density and a potassium hydroxide electrolyte. Alkaline batteries have a nominal voltage of 1.5 V and operate over a wide temperature range (approximately -20 to 70.degree. C.). Furthermore, they are capable of withstanding and functioning under severe operating conditions, such as mechanical shock, high pressure and the like.
The demand for rechargeable or secondary batteries of high energy density and specific energy has, however, increased with the increasing demand for portable electronic equipment, e.g. cellular phones, laptop computers, consumer electronics, power tools, etc. In order to meet this demand, various types of rechargeable batteries have been developed including improved nickel-cadmium aqueous batteries, various formulations of aqueous nickel metal hydride batteries and non-aqueous rechargeable lithium batteries (hereinafter referred to as "Li-ion secondary batteries"). Nevertheless, all secondary battery chemistries are more expensive than primary battery chemistries. The Li-ion secondary battery chemistry is particularly expensive.
Nickel-cadmium aqueous batteries are the least expensive among the portable rechargeable battery chemistries mentioned above. A typical nickel-cadmium secondary battery (having a nominal voltage of about 1.2 V) includes a metallic cadmium sponge anode, a nickel hydrate (Ni(OH).sub.3) based cathode and an electrolyte, which may have an aqueous solution of potassium hydroxide (KOH). Some advantages of this chemistry include long life (with up to 1500 charge discharge cycles), capability of high discharge rate, and quick charge times, for example. Nickel-cadmium batteries are particularly attractive in applications where low battery cost is of paramount importance and that require high discharge rates, which are not provided by other chemistries. Unfortunately, the use of cadmium in the nickel-cadmium chemistry is a growing environmental concern.
Nickel metal hydride is a rapidly growing alternative secondary battery chemistry. The primary difference between the nickel-cadmium battery chemistry and nickel metal hydride battery chemistry is that in the nickel metal hydride chemistry, the active material in the cathode is a hydrogen absorbing metal alloy, which reversibly absorbs and releases hydrogen gas generated by the electrochemical discharge reaction. Upon absorption of hydrogen, the metal alloy becomes a metal hydride. The advantages of nickel metal hydride chemistry over nickel-cadmium chemistry are the provision of higher energy densities and lack of cadmium-related environmental problems, for example. Nickel metal hydride is, however, more expensive relative to the nickel-cadmium chemistry and less expensive relative to the Li-ion chemistry.
Among the commercially available portable rechargeable battery chemistries, Li-ion provides the highest energy densities. A typical Li-ion cell consists of a lithiated cobalt oxide, lithiated nickel oxide or lithiated manganese oxide based composite cathode, a carbon-based anode, and a lithium ion liquid electrolyte. Lithium is the lightest and is the most electropositive alkali metal (charge capacity 3.86 Ah/g). Furthermore, Li-ion batteries have a long cycle life, provide a high cell voltage and are capable of functioning at a wide temperature range. Unfortunately, Li-ion battery chemistry is the most expensive of the portable rechargeable battery chemistries and is inappropriate for applications requiring rapid discharge. Furthermore, during recharging of the Li-ion batteries, dendrite formation undesirably leads to shorts. Further still, Li-ion batteries are typically equipped with electronic circuits to monitor the discharge history and prevent overcharging. This protective circuitry adds to the cost of manufacturing a battery. Further still, it is expensive to produce the high quality porous electrodes that are employed in the Li-ion battery.
Lithium-polymer (Li-polymer) battery chemistry, which is a modification of the Li-ion battery chemistry, is another emerging secondary battery chemistries for portable electronics. Li-polymer chemistry is similar to the Li-ion chemistry, except that a polymer or gel electrolyte, e.g., a poly(vinylidene fluoride) (PVDF) electrolyte containing lithium salt, is used in place of a liquid electrolyte. Li-polymer chemistry is the chemistry of choice over Li-ion in portable electronic applications because the Li-polymer battery can be processed into a thin sheet whereas Li-ion batteries require a container for holding the liquid electrolyte. Thin sheet processability and deformability are ideal for portable electronic applications where space is at a premium. The elimination of the liquid electrolyte also improves safety characteristics over Li-ion cells, which use an inflammable liquid electrolyte. Unfortunately, the Li-polymer batteries also suffer from some of the same drawbacks as the Li-ion battery mentioned above, e.g., added costs of overcharge monitoring circuits and producing a high quality porous electrode, and dendrite formation.
In order to circumvent the dendrite formation in the Li-ion and Li-polymer battery mentioned above, Li-ion and Li-polymer battery chemistries typically employ Li insertion compounds instead of Li metal as the anode material. In this case, dendrite formation is avoided as long as the potential between the two electrodes never reaches Li/Li.sup.+. Li insertion compounds are known to have open structures that are capable of accepting and releasing Li.sup.+ ions. By way of example, one such lithium insertion compound for the anode is Li.sub.x C.sub.6, which has a charge capacity for x=0.5 of about 0.186 Ah/g and is well known in the art for accepting large quantities of lithium. Unfortunately, the performance of the cathode (e.g., lithiated cobalt oxide, lithiated nickel oxide or lithiated manganese oxide based composites) and anode materials is dependent on their morphology, which controls the ionic conductivity. Thus, in order to have a sufficiently high ionic conductivity in the presence of negligible bulk ionic diffusion, high quality porous electrodes are necessary. Consequently, even with the use of lithium insertion compounds, the drawbacks of significant costs associated with high quality porous electrodes and electronic circuitry for protection against overvoltage still persists.
The search for a commercially viable battery technology for portable electronic applications is knocking at the door of battery technology that employs a conductive polymer electrode and is still very much in its infancy. The conductive polymer typically include conjugated polymers, which are known like other polymers to undergo p-type doping with good reversibility. By way of example, typical upper limits for anion intercalation (p-type doping) of lower energy gap polymers (i.e. having an energy gap of approximately 2 eV) is about 0.25 electrons per repeat unit and of higher energy gap polymers (i.e. having an energy gap higher than approximately 3 eV) is about 0.3 to about 0.5 electrons per repeat unit. The typical upper limits for cation intercalation (n-type doping) are lower than the upper limits for anion intercalation mentioned above. Furthermore, the charge capacity of the conjugated polymers are lower than the corresponding theoretical values for Li metal and Li intercalation anodes mentioned above.
Bridgestone Lithium Polymer Button Cell (hereinafter referred to as "button cell"), available from Bridgestone Corporation of Tokyo, Japan, is one polymer electrode battery that is commercially available. It employs a LiA1 (lithiated aluminum) anode, polyaniline cathode and a liquid electrolyte. It is a supercapacitor and therefore high charge capacity is not essential. The button cell, for example, has a nominal charge capactiy of about 4 mAh and is designed for &gt;10.sup.3 shallow cycles at 1 mAh.
Unfortunately, the Bridgestone button cell fails to meet the power demands of most portable electronic equipment. The button cell is employed in a limited role as an electrical buffer in electronic circuitry. By way of example, in solar cells the button cell may function as a support cell when the main cell for energizing the circuit is inactive. In other words, the button cell fails to fully exploit the advantages of polymer-based systems to increase the energy density of the battery, which would make it suitable for portable electronic equipment applications.
Furthermore, the button cell has a polymer electrode that is made from electrochemically synthesized polyaniline, which is undesirable for large scale fabrication of electrodes. Further still, polyaniline film produced electrochemically is brittle and tends to break when subject to external force. Consequently, polyaniline electrode fails to lend itself to thin film processability.
What is therefore needed is a lower cost battery chemistry that has performance comparable to commercially available chemistries and offers thin sheet processability and deformability.